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Answer:

• Sial is composed of silicon and aluminium.
• It is upper layer which forms a discontinuous cover over the Earth's crust and is entirely absent the ocean floor.

• Sima is composed of silicon and magnesium.
• This is the second layer below sial which forms the ocean base.

Explanation:

The name 'sima' was taken from the first two letters of silica and of magnesia. Comparable is the name 'sial', which is the name for the upper layer of Earth's crust (continental crust).

In geology, the term sial refers to the composition of the upper layer of Earth's crust, namely rocks rich in aluminium silicate minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but 'sial' is a geochemical term rather than a plate tectonic term